It is known that certain materials, which exist in a stable condition at room temperature, in either a substantially amorphous or substantially crystalline state can be converted from one of these states to the other by supplying sufficient energy to heat the material to its melt temperature and then allowing it to cool under controlled conditions to a crystalline or an amorphous state.
It is also known that such materials may be used for storing and retrieving information. These materials may be used in the form of thin films for optical recordings. The thin films are written upon selectively changing particular areas of the thin film from one state to the other (from amorphous to crystalline or crystalline to amorphous). Such change may be accomplished by providing a low amplitude energy pulse for sufficient duration to heat the material to just below its melting point after which the film slowly cools in the crystalline state. On the other hand to convert the film from a crystalline to an amorphous state, rapid cooling is essential. This change, may be accomplished by pulsing the film with a high energy pulse source to raise the film to its melting point. The film must then be rapidly cooled to freeze the film in the amorphous state before crystallization can occur.
One of the most difficult challenges is to develop optical recording elements which possess both fast erasure speed and long data retention time. Both of these desirable features relate to the transition of the film from the amorphous phase to the crystalline phase. To have fast erasure speed requires the crystallization rate to be exceedingly fast at the elevated temperatures produced by a laser pulse. Long data retention time, on the other hand, requires exceedingly slow crystallization rate during keeping near room temperatures.
Most studies have been concentrated on films of tellurium rich (Te&gt;85%) alloys. For these materials, however, long erasure time (&gt;1 .mu.s) is needed unless data retention time is sacrificed.
Recently, Chen et al., in a paper entitled "New Ideas for Phase-Change Media-Achieving Sub-Microsecond Erase With Data Stability", presented Oct. 15-17, 1985, at the Topical Meeting on Optical Data Storage, IEEE and OSA, in Washington, D.C., has reported achieving a significant improvement in performance by using nearly stoichiometric GeTe films. However, such films possess limited corrosion resistance and data stability.